The invention relates to a two circuit braking system for vehicles, and more particularly to a master brake cylinder for such a system. The master cylinder includes an auxiliary piston within which slides a control piston. When suitably displaced, the control piston opens a control valve disposed between a source of brake pressure and a first braking circuit. The pressure within the first braking circuit reacts back onto the control piston and also onto the control surface of a piston functioning in the main cylinder intended for the second braking circuit. A master brake cylinder of this type is described in the German Offenlegungsschrift No. 2 312 641. In the apparatus of that publication, after the control piston has been slightly displaced, a flat seat valve lying between a source of pressure and the first brake circuit is opened but this opening also requires the opening of a slide valve. The pressure fed to the second brake circuit also affects the control surface of the piston for the second brake circuit and displaces it.
The known apparatus also includes an auxiliary piston which is displaced by the pedal when the source of pressure fails and which thus creates the brake pressure for both circuits. The apparatus further includes mechanical means passing through the auxiliary piston and providing a displacement of the piston for the second brake circuit when the pedal is actuated but when the first circuit leaks.